This invention relates to apply pressure controls for a friction torque transmitting device, and more particularly, to apply pressure controls for a friction torque transmitting device having directional sensitivity.
In the prior art, directional sensitivity in a friction torque transmitting device, such as a clutch or brake, is achieved by incorporating a one-way torque transmitting device in parallel with a friction torque transmitting device. With these devices, it is possible to selectively eliminate or provide engine coast braking during a vehicle coast condition. If the friction torque transmitting device is not engaged, engine braking will not be available and vice versa. These controls require the operator to manually select a position at the manual shift lever to obtain coast braking.
If directional sensitivity is desired in a higher drive ratio; i.e., other than first gear, it is necessary to have a friction torque transmitting device in series with a one-way torque transmitting device as well as a second friction torque transmitting device disposed in parallel drive relation therewith. The series friction device is utilized during a ratio interchange while the parallel friction device is energized when engine coast braking is desired.
With these prior art devices, at least one friction device and a one-way torque transmitting device are necessary to provide selective coast braking. Obviously, these devices require the allocation of axial space and increase the cost of manufacture. The use of selective coast braking or reverse torque transmittal does improve shift quality and perceived performance during normal upshift ratio change operations. To this end, such devices are considered to add value to the transmissions.
It is also known to provide a fluid operated friction torque transmitting device wherein a pair of opposing chambers are provided such that on torque reversal, fluid pressure is directed to both chambers to reduce the torque capacity of the friction device. The additional fluid chamber is of smaller area than the normal apply chamber, such that the friction device is always available to transmit some reverse torque. Such a device is shown in U.S. Pat. No. 4,805,752 issued to Malloy et al. on Feb. 21, 1989, and assigned to the assignee of the present invention. In Malloy et al., there is discussion of two other types of direction sensitive devices which utilize a fluid operated friction device. In these devices the apply chamber is exhausted during torque reversal thereby eliminated coast braking. Assemblies of such devices are shown in U.S. Pat. No. 3,734,251 issued to Annis et al. Feb. 22, 1973, and U.S. Pat. No. 3,410,370 to Maurice Nov. 12, 1968.